Endangered cactus restoration: mitigating the non-target effects of a biological control agent (Cactoblastis cactorum) in Florida

Summary

Study 1

The natural distribution of the cactus moth Cactoblastis cactorum is Argentina, Paraguay, Uruguay and southern Brazil. It was released as a biological control in the 1950s and 1960s to several Caribbean islands to control Opuntia spp. cacti. The first US record of cactus moth came from Florida in 1989. It was thought that the moth had either dispersed from Cuba or had been brought in on cacti imported from Haiti. In the USA it now threatens native cacti and concern is greatest for the endangered semaphore cactus, Opuntia corallicola, of which only two wild populations exist. Planting trials were therefore undertaken in an attempt to bolster semaphore cacti numbers and to test the effectiveness of different planting treatments to protect it from cactus moth. In this study, whether semaphore cacti planted near prickly pear O.stricta (a common cactus which hosts cactus moth) would be attacked by Cactoblastis and, if so, whether they would survive, was tested.

Study site: Whether semaphore cactus Opuntia corallicola planted near prickly pear O.stricta cacti would be attacked by cactus moth Cactoblastis cactorum, and the effect of planting in the sun versus shade on cactus growth, nitrogen levels, attack by cactus moth and other mortality was tested on the Saddlebunch Keys, Florida, southeast USA. This 'Associational Susceptibility Experiment' ran from 1998 to 2003.

Experimental design: In 1998, 180 small (< 30 cm tall) semaphore cacti (cultivated, from wild collected cactus pads, at the University of South Florida) were divided into 18 replicates of 10 plants each, and planted in two rows of five, 20 cm between plants, with 50–100 m between each replicate. Eight replicates were planted in the shade of buttonwood trees Cornocarpus erectus and 10 in full sunlight. Six of the replicates were planted within 5 m of prickly pear infested with Cactoblastis, six within 5 m of non-infested prickly pear, and six between 20 - 30 m from any prickly pear. Cacti were surveyed every 3 months for 5 years, until 2003. Height and mortality were recorded. After 5 years, average height of the surviving cacti in each replicate was used to compare growth between sun and shade treatments. Plant quality was estimated by removing one small pad from one of the cacti in each replicate after 2 years of growth and analyzing for nitrogen.

Causes & characteristics of semaphore cacti mortality : Semaphore cacti killed by Cactoblastis exhibited a hollow appearance as all tissues between the epidermal layers were removed, usually characteristic frass (droppings) or live caterpillars within cacti were also present. Many plants also rotted at the crown, with the fungi Fusarium oxysporum present in the rotted crown area. A few cacti were knocked over, uprooted or destroyed by falling branches.

Semaphore cacti mortality rates: In total, 128 of our 180 planted cacti died over the 5 years of the experiment (total mortality = 71.1%). The cacti died at a fairly constant rate of between 14 to 15% per year. The greatest source of mortality was from Cactoblastis, which killed significantly more cacti in shaded conditions (68%) than in full sun (28%). There was no effect of prickly pear proximity on attack rates. The only other cause of mortality in the experiment was crown rot, though crown rot did not significantly differ between treatments (see Table 1, attached). Semaphore cacti did not grow significantly more in the shade (18.2 cm ± 14.4 SE) than in the sun (12.0 cm ± 8.3). Plant nitrogen content was significantly higher in the shade (%N = 1.34 ± 0.06) than in the sun (%N = 1.22 ± 0.07).

Conclusions: Semaphore cacti planted between 20-30 m away from the prickly pear (which act as a reservoir for cactus moth) were just as frequently attacked and killed by Cactoblastis as semaphore cacti planted within 5 m. In addition, Cactoblastis attack was greater in the shade than in the sun, possibly because shaded plants had higher nitrogen content.

Study 2

The natural distribution of the cactus moth Cactoblastis cactorum is Argentina, Paraguay, Uruguay and southern Brazil. It was released as a biological control in the 1950s and 1960s to several Caribbean islands to control Opuntia spp. cacti. The first US record of cactus moth came from Florida in 1989. It was thought that the moth had either dispersed from Cuba or had been brought in on cacti imported from Haiti. In the USA it now threatens native cacti and concern is greatest for the endangered semaphore cactus, Opuntia corallicola, of which only two wild populations exist. Planting trials were therefore undertaken in an attempt to bolster semaphore cacti numbers and to test the effectiveness of different planting treatments to protect it from cactus moth. These planting trials were undertaken to determine whether plantings distant from other cacti and the use of moth exclusion cages would improve semaphore cactus survival.

Study site: It was thought that association with prickly pear Opuntia stricta (a cactus mothCactoblastis cactorum host) would make semaphore cactus O.corallicola more susceptible to cactus moth attack, thus these planting trials were undertaken to determine whether plantings distant from other cacti, and the use of moth exclusion cages, together would improve semaphore cactus survival. These trials were undertaken on Little Torch Key, Florida, southeast USA.

Experimental design: The 'Cactoblastis Exclusion Experiment' ran from 1996 to 2003. In 1996, 96 small (< 30cm tall) plants were planted at least 500 m from known prickly pears. A 2 × 2 factorial design was used to test the effectiveness of cages at protecting semaphore cacti from Cactoblastis with planting in the shade or in full sunlight as an additional treatment. Cages comprised 60 × 60 × 60 cm wooden frames overlain with mosquito netting, with hinged lids to allow access. For the cage treatment, cacti were planted in groups of four, one in each cage corner, and for the no-cage treatment one cactus was placed at each corner of a 60 × 60 cm plot on similar substrate. In total there were 12 replicates of caged cacti, 12 of uncaged cacti, and in each of these treatments six were placed in the sun and six in the shade.

Cacti were visited every 3 months for 7 years, until 2003, and height and causes of death were recorded. After 3 years, average height of the surviving cacti in each treatment was compared.

Semaphore cactus growth & survival: Cages did not affect cactus growth and semaphore cacti also did not grow significantly more in the shade (6.1 cm ± 7.4) than in the sun (2.5 cm ± 1.4). There was no mortality by Cactoblastis in any of the treatments. However, some cacti were knocked over and uprooted by animals, probably key deer Odocoileus virginianus clavium. The mosquito-net cages, designed to prevent attack by Cactoblastis, unfortunately attracted the attention of some larger mammals, such as key deer which rubbed against and broke them, and sometimes trampled cacti to death. This resulted in a significant difference in trampling death between caged (21% of plants) and uncaged (4%) treatments. Cage breakage, and the resultant trampling of cacti, killed more cacti in sunny areas, where animals could access all cages unhindered, compared to those in shaded conditions where access was more restricted by branches. Storm-induced branch breakage crushed and uprooted some cacti, but such mortality was minimal as most fallen branches were small (4% in uncaged plots, 0% in cages as these effectively protected the cacti). In this study the most important source of mortality was crown rot, though crown rot was not significantly affected by treatments (65% mortality in cages, 73% uncaged). In this experiment, 80 of 96 cacti died over 7 years (83.3% mortality).

Conclusions: Attack from Cactoblastis was successfully avoided (regardless of whether using protective cages or not) for semaphore cacti planted at least 500 m from prickly pear plants. Cages attracted the attention of deer, which sometimes destroyed the cages and trampled the cacti inside to death. Crown rot caused high mortality in this planting. These trial results suggests that it would be pertinent in future planting of semaphore cacti to ensure that individuals are at least 500 m away from prickly pear plants (which may act as a reservoir of Cactoblastis). Replicated plantings near to and far away from prickly pear are recommended by the authors to fully test this.

 

Study 3

The natural distribution of the cactus moth Cactoblastis cactorum is Argentina, Paraguay, Uruguay and southern Brazil. It was released as a biological control in the 1950s and 1960s to several Caribbean islands to control Opuntia spp. cacti. The first US record of cactus moth came from Florida in 1989. It was thought that the moth had either dispersed from Cuba or had been brought in on cacti imported from Haiti. In the USA it now threatens native cacti and concern is greatest for the endangered semaphore cactus, Opuntia corallicola, of which only two wild populations exist. These planting trials were undertaken to determine whether fertilizing cacti promoted better growth and survival, especially via resistance to crown rot, which had caused high and significant mortality in earlier planting trials (see Cases 537 and 538).

Study site: The semaphore cactus Opuntia corallicola NPK fertilization experiment was undertaken on six Florida Keys: No Name; Big Pine; Little Torch; Cudjoe; Sugarloaf; and Ramrod, and ran from 2000 to 2003.

Experimental design: It was thought that association with prickly pear Opuntia stricta (a cactus moth Cactoblastis cactorum host) made semaphore cactus O.corallicola more susceptible to cactus moth attack, thus following on from results of earlier trials, the effect of Cactoblastis was minimized by planting semaphore cacti 500 m away from prickly pear. Earlier trials also indicated that semaphore cacti also had a tendency to grow slightly better in shade, and hence all cacti were planted in the shade of tropical hammocks survival in light gaps rather than deep shade compared. As crown rot had also been observed to consistently cause high mortality, whether fertilizing cacti promoted better growth and survival, especially via resistance to crown rot, was tested.

At each of the six separate Keys, 40 cacti were divided into four replicates of 10. One of four treatments was applied to each replicate:

1) full shade of a tropical hammock and 250 g of 6-6-6 (NPK) fertilizer added once a year in the spring

2) light gaps with fertilizer

3) light gaps with no fertilizer

4) shade with no fertilizer

Average height of the surviving cacti in each replicate over the 3-year period was analyzed, and death by crown rot and falling branches was compared between treatments.

Semaphore cactus growth & mortality: Mortality in this experiment was again high (see also Cases 537 and 538) with 108 cacti (45%) dying within 3 years. Cactoblastis killed no cacti, most succumbing to crown rot. It was hoped that fertilizer addition might infer some crown rot resistance but there was no significant decrease in the fertilized treatment (42.5% ± 9.1) compared to the control (37.5% ± 9.2). There was also no effect of light gaps on crown rot (shade, 44.2% ± 8.8; light gaps, 35.8% ± 9.3). There were few deaths from falling branches uprooting cacti, but there was a tendency for more death from falling branches in shade (8.3% ± 3.2) than light gaps (1.7% ± 1.1).

For surviving cacti, gain in height during the first 3 years was significantly higher in fertilized treatments (35.7 cm ± 6.1) than in control plots (18.7 cm ± 3.5). Growth was similar in light gaps (28.8 cm ± 6.2) and in the deep shade of hammocks (25.4 cm ± 4.8). Some replicates had 100% cactus death.

'Volunteers' (cactus pads appearing at the base of parent plants) grew at about the same rate as the cacti planted in the cage experiment (Case 538), and by the end of 3 years, growth averaged 7.0 cm ± 0.9. However, mortality appeared to be much lower among volunteers than among planted cacti, and after 4 years only 22.3% ± 4.2 had died, all from crown rot.

Conclusions: Planting semaphore cacti at least 500 m away from prickly pear appeared to negate the potential problem of mortality due to Cactoblastis. NPK soil fertilization greatly stimulated cactus growth but did not affect cactus death from crown rot.

 

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